The full-length cDNA for a new plasma sialoglycoprotein (sgp120) was elucidated from human fetal liver cDNA libraries by standard cloning and sequencing technologies. We have demonstrated that several forms of the gene that codes for sgp120 exist due to spliced variants. Sgp120 was first identified by its capacity to interact with the activated fourth component (C4b) of the classical complement pathway (CCP). Results support the idea that this form of the protein, sgp120-A, regulates the activation of the CCP. Sgp120 is also cleaved by kallikrein of the intrinsic coagulation-kinin generating pathway to produce protein fragments that may possess biological activity. The data supports our hypothesis that the larger C4b binding form, sgp120-A, is a product of a spliced gene. Pigtryin (PT), an identified acute phase protein and postulated analog of human inter-alpha-trypsin inhibitor (IATI) was shown to have a peptide match of 76% identity with 81% similarity with sgp120. Thus, PT is most likely the pig analog of human sgp120 and not IATI. Using a sensitive ELISA for sgp120 we have identified 2-3 fold increases in the serum sgp120 levels of burn patients suggesting that sgp120 like other complement components is an acute phase reactant. Sgp120 also shows strong homology to each of the three mature heavy chains of IATI. Homology to HC3 was most significant with 51% identity and 71% similarity over the 5' 619 amino acid residues upstream from the splice region.